1. Field of the Invention
The present invention relates to an acoustic wave sensor using a piezoelectric element and a surface acoustic wave, and more particularly to an acoustic wave sensor capable of realizing a dual mode such that it can operate in a gaseous or liquid environment.
2. Description of the Related Art
A piezoelectric element can convert electrical energy to mechanical energy and can also convert mechanical energy to electrical energy, and is being widely used as a component of a sensor or an actuator utilizing such characteristics. The piezoelectric element may be used in vibration sensors, acceleration sensors, angular velocity sensors, flux sensors, pressure sensors, or the like, and, in recent years, is being widely used for a biochemical sensor due to the development of a technology for forming a detection layer containing a biochemical material (e.g. antibody, aptamer, etc.) on a portion of its surface and the related deposition technologies.
The piezoelectric element also may be used as an acoustic wave sensor configured to detect minimum amounts of material through a principle in which the operation frequency or resonance frequency of the element is changed by a minute mass of a target which is introduced from the outside and captured by a detection layer on a surface of the element. Such acoustic wave sensors are classified into a Surface Acoustic Wave (SAW) filter having an excellent sensitivity and a Surface Skimming Bulk Wave (SSBW) filter.
FIG. 1A is a diagram illustrating a general structure of an acoustic wave sensor using a surface acoustic wave filter. Referring to FIG. 1A, the acoustic wave sensor 100 includes a quartz substrate 110 having piezoelectric characteristics, an input electrode 111 formed on the quartz substrate and through which an RF signal is input, an output electrode 112 for detecting an output frequency change due to a change in mass, and a sensing film 113.
An RF signal is applied to the acoustic wave sensor from the outside through the input electrode 111, and the operation frequency change by the minute mass of a material (a gas or a liquid) applied to a surface of the sensing film 113 is output to the output electrode 112 in order to detect a target.
The acoustic wave sensor 100 uses a change in frequency accompanied when a gaseous or liquefied target is detected on a surface of the sensing film 113, and the relation between a frequency change and a mass change may be expressed by Equations (1) and (2).
                    f        =                  v          λ                                    Equation        ⁢                                  ⁢                  (          1          )                    where f indicates a frequency, λ is the wavelength of an acoustic wave applied to a surface of the sensing film 113, and v is the velocity of the acoustic wave. The velocity of the acoustic wave may be expressed by Equation (2).
                    v        =                              c            ρ                                              Equation        ⁢                                  ⁢                  (          2          )                    where v indicates the velocity of the acoustic wave prior to the acoustic wave sensor, c is the stiffness of a medium through which the acoustic wave proceeds, and ρ is the mass density of a surface of the acoustic wave sensor. It can be seen from Equation (2) that the velocity of the acoustic wave changes according to change in the mass and from Equation (1) that the frequency is determined according to a change in the velocity of the acoustic wave.
As a result, the acoustic wave sensor 110 can detect a target when the mass of the target changes the velocity of an acoustic wave and hence the frequency of the acoustic wave.
The acoustic wave sensor 100 can generate a SAW or a SSBW frequently called a Rayleigh wave according to the cutting direction of the quartz substrate 110.
A Rayleigh wave and a surface SSBW are similar to each other in that most of acoustic energy is concentrated on their surfaces but are much different from each other in a physical aspect. When a Rayleigh wave is assumed to propagate in the z axis on a surface of a medium in FIG. 1C, there exists a vibration component in the y axis direction which is an upward and downward direction with respect to the surface of the medium. Therefore, a Rayleigh wave extinguishes since a vibration component in the y axis uses energy to generate a longitudinal wave when it contacts with a liquid, while the Rayleigh wave is sensitive to change in the state of a surface. On the other hand, when a SSBW is assumed to propagate in the z axis in FIG. 1C, since it is a shear horizontal wave having no vibration component in the y axis and having only a medium vibration component in the x axis, even when a surface is in a liquid environment, the SSBW does not generate a longitudinal wave into the liquid, preventing loss of energy and continuously maintaining its waveform. That is, while a Rayleigh wave has a high sensitivity and can detect of a target material captured by a surface of a sensor, the Rayleigh wave is used mainly to detect a gaseous target in a gas environment since it's signal is rapidly damped in a wet or liquid environment.
Although a surface skimming bulk wave is an acoustic wave which propagates into a medium from a surface of the medium at a minute angle and cannot be referred to a surface acoustic wave strictly, it is named a Pseudo-Surface Acoustic Wave (P-SAW) or a Leaky Surface Acoustic Wave (LSAW) or is often classified as a surface acoustic wave. Since the surface skimming bulk wave is less sensitive than a sensor utilizing Rayleigh wave due to dispersion of acoustic energy on a surface of a medium into the medium, but still has a high sensitivity as compared with other types of acoustic wave sensors in which energy is dispersed to an entire medium and maintains the waveform of its signal without damping the signal even in a liquid environment, the SSBW is widely used in a wet environment or for detection of liquefied targets.
That is, a sensor using a Rayleigh wave is suitable for a gas detecting sensor since a Rayleigh wave provides a performance extremely sensitive even to ppb, and a sensor using a surface skimming bulk wave may be used in a wet environment or for a liquid detecting sensor since a SSBW even endures a liquid environment well. Hereinafter, a Rayleigh wave is referred to as a surface acoustic wave for convenience.